All eyes on the zebra’s stripes

Richard Collins asks what is the purpose of a zebra’s colouring?

ZEBRAS are horses in prison clothes. People sometimes ask if their coats are black with white stripes or white with black ones. Hair is naturally white, whereas black pigment must be produced in the animal’s body. The underlying zebra colour is, therefore, white. But the really intriguing question about the stripes is why they exist at all? Nobody is sure of the answer to this question but there are theories.

The most obvious suggestion is that the stripes make life difficult for the zebra’s enemies. Lions are the animal’s greatest threat. A pride, hunting together, selects a potential victim, usually a vulnerable individual, before launching a co-ordinated attack. As soon as the predators are spotted, the zebras flee. The lions must then keep their target animal in sight, separate it from the crowd and pursue it to exhaustion.

Cats are colour blind and, so the theory goes, the fast moving mass of stripes makes tracking an individual animal extremely difficult. It’s a plausible theory, but if stripes are so effective why don’t other prey animals have them? Deer antelope and wildebeest are targeted by lions, cheetahs and hyenas, but they have no such markings. Is there more to stripes than meets the eye?

When watching zebras in Africa, a wildlife ranger told me that the stripes are the animal equivalent of a supermarket bar code. A mare leaves the herd to give birth and its mother is the only animal a newborn baby sees. While she walks around her baby, encouraging it to stand, the foal learns her bar code. On its feet in minutes, the youngster follows its mother into the herd. Being able to recognise the parent in the confusion of a large and often fast-moving throng is essential; no zebra will feed a youngster which is not its own.

The bar code idea is attractive but scientists, to whom I mentioned it, seemed baffled. One asked if the idea had been tested experimentally and, if so, how. Smell, he pointed out, is the really important sense among members of the horse family and foals in a herd should be able to find their mothers by scent alone.

A display in Berlin’s natural history museum offers another explanation for zebra stripes. They are, it claims, a defence against enemies, but not big cats and hyenas. Zebras, like all hoofed animals, are tormented by flies. The tsetse, one the great scourges of Africa, looks like a common house fly with projecting mouth parts. Tsetses feed on the blood of animals and people.

Parasites, which cause deadly diseases, are carried by the flies from one victim to the next. Sleeping sickness, which affects humans, begins with a fever. As the disease develops, the victim experiences disorientation, drowsiness during the day and sleeplessness at night. Coma follows which, if the condition is not treated, leads to death. The animal form of the disease is known as nagana.

A fly’s eye is not like ours. The image, which the insect sees, is a composite derived from hundreds of mini eyes. The creature has 360° vision but resolution into individual objects is poor.

To a fly, the world is a patchwork of coloured blotches. A tsetse will target any large area of uniform colour as it searches for a victim. The zebra’s stripes break up colour patches and so the fly doesn’t spot the animal. The theory seems a bit implausible but support for it comes from a close relative of the zebra.

The quagga is the only large mammal to have become extinct in Africa in the last 200 years. This species of zebra is known only from pictures, skins and a few museum specimens.

The animal had stripes on the neck and front of its body but there were none, or very faint ones, over the rest of its coat. The quagga’s ancestors evidently had stripes but they were discarded. Why?

The vital factor, it’s suggested, is that quaggas lived only in areas where there were no tsetses, so stripes were no longer needed.

However, whether stripes are there to fool big cats or flies, the objection to the theory still holds; why have other hoofed animals failed to develop similar colour schemes?